Richmond dispatch. [volume] (Richmond, Va.) 1884-1903, 1887-08-21

,orare foot. This force operates to burst
ont The walls of building. 1t is not
;mnrobable that the diminntion OF
pressure may be much greater than
this, but even the amount named Is
sufficient 70 account for the bursting
ont Of the frail-walled structures which
these devastating movements encounter
in the western parts OF the United
States.
The way in which these tornado
vbirls are formed differs IN certain
essential particulars from the way in
which wbirlwinds are created, as has
been well shown by Professor Ferrel.
The most important points OF differ
ence are as follows: The dust-whirls
are due to the heating of a thin layer o.
air next the ground. The small mass
of this layer prevents its upward whirl-
ing from bringing about any power
full movements of the atmosphere.
In the tornado the heat OF
the lower air has different origin.
When a cyclone passes over the
surface Of the country certain peculiar
movements Of the atmosphere which it
it produces brings large volumes OF the
warm and moistened air to the earth's
surface and oyerlay them by cool
stratum. This layer of warm, moist
air tends to rise up for the same reason
that the thin layer of dry air which
forms the dust-whirlis impelled upward,
but on account of its great mass the in
tensity of the upward urgence is far
greater. In the sand-ghirl, the upward
motion being close TO the earth'ssurface,
for the reason that the stratam which is
impelled upward is very thin, but in
tie tornado the stratum of heated air
is usually about 1000 feet thick
therefore It's whirling action naturally
originates at the upper surface of the
hot layer, for it is at that point the up
ward motion begins. Starting in this
upper region, the whirl extends pro
gressively downward. just as in the
bath-tub the whirl extends progress
lively upward from the point at
which the motion originated until
the vhirl may touch the sur
face of the earth. When these
whirls begin they only involve snail
part Of the air about the point of origin,
and so the acquired velocity of the par
ticles when they come to the centre is
not great; but gradually they suck
air from farther and farther away. As
tie field of supply becomes larger, and
the particles move from a greater dis
lance, they approach that centre with
greater and greater speed, and the spi
raj widens and turns with accelerated
velocity. The longer the journey of
the particle, the swifter its whirling
motion becomes.
Fortunately the paths of Tomatoes
are ordinarily very narrow-the widest
have diameter Of less than two miles,
the narrowest of only forty feet. Ia
most cases q tornado is seriously de-
structive over n width not exceeding
500 feet. The length of the tornado's
path across the country does not com
monty exceed thirty niles, aud itgenc-
rally trayersos the distance in about an
hour. When the upward corkscrew
motion of the outer part OF the
spiral and the swiftor uprush of the
air through the central shaft haze
drained away the most OF TIE
warm sir which gave birth to
tie motion, the tornado dies away.
Tie equilibrium of the air-mlsses is
for a time restored. the heavier air has
fallen down upOn the surface. and the
warm air, spreading later4lly rs it At
tains the level to which it tends, comes
into n state OF quiet. Assuming the
width Of the destruction brought about |
by the storm at GOO feet and the length |
of its journey at Thirty miles We find
hat the area OF its devastation amounts
10 about 2,O0C acres, or to square area
lbout two miles on a side. Over tai,
irea the destruction is ordinarily more |
than that which occurs in the !
most severe earthquakes.